An aqueous resin, which is water soluble or water dispersible, is being used in various fields including paints, ink, fiber treating agents, adhesives, coating agents and the like.
The water soluble or water dispersible aqueous paint uses an aqueous medium, and thus is free of risk for environmental pollution and fire, and cleaning of painting equipments, such as a brush, a roller and a spray gun, and removal of contamination with the paint may be performed with water. Owing to these factors, there are increasing demands of the aqueous paints in recent years.
The aqueous resin generally has a carboxyl group introduced thereto for imparting water solubility or water dispersibility to the resin itself. Accordingly, the carboxyl group remaining in the coated film may induce hydrolysis, which may impair the strength, durability and appearance of the coated film.
As a measure for enhancing the properties including the strength, water resistance and durability of the coated film of the aqueous resin, such as method is ordinarily employed that an external crosslinking agent, such as an aqueous melamine resin, an aziridine compound and a water dispersible isocyanate compound, which is capable of forming a crosslinked structure through reaction with the carboxyl group, is used in combination.
However, the crosslinking agent may be difficult to use in some cases due to toxicity, reactivity and the like. Specifically, the crosslinking reaction with the crosslinking agent proceeds while consuming the carboxyl group, and therefore, the strength, water resistance, durability and the like of the coated film may be enhanced by reducing the carboxyl group, but the unreacted crosslinking agent remaining if any may cause toxicity in the coated film. When the unreacted carboxyl group remains in the coated film, on the other hand, the water resistance and durability of the coated film may be lowered. Accordingly, various problems may occur in the case where the crosslinking agent and the carboxyl group in the aqueous resin are not completely reacted.
For solving the problem of toxicity, a carbodiimide compound is receiving attention in recent years. For example, Patent Document 1 discloses aqueous dicyclohexylmethanecarbodiimide that has good reactivity and storage stability and convenient handleability as a crosslinking agent for an aqueous resin.
The aqueous dicyclohexylmethanecarbodiimide has no toxicity and has sufficient pot life.
Patent Document 2 discloses a water soluble or water dispersible carbodiimide compound capable of enhancing water resistance, solvent resistance and adhesiveness, which is formed by, upon hydrophilizing the carbodiimide compound, introducing methyl glycolate or methyl lactate, which has a structure close to the aqueous urethane resin and the aqueous acrylic resin, into the terminals thereof.